CN111819605A

CN111819605A - Passive optical sensor

Info

Publication number: CN111819605A
Application number: CN201980017162.7A
Authority: CN
Inventors: M·祖阿尔法哈里; M·费舍尔; I·尼尔德; M·威廉森
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 2018-03-23
Filing date: 2019-03-18
Publication date: 2020-10-23
Anticipated expiration: 2039-03-18
Also published as: CN111819605B; WO2019179951A1; US20210005078A1; EP3769291B1; EP3769291A1; US11962293B2

Abstract

A passive sensor (1) for detecting operation of a light source (9) includes a light sensitive sensor (3) which, when illuminated by the light source (9), supplies a voltage to a field effect transistor (4) which can switch a circuit (46) to control a device at a remote location. The fet switches rapidly between a very low resistance (short circuit) and a very high resistance (open circuit), thereby allowing a binary indication to be given in the circuit (46). The circuit (46) is used to charge the electrical storage device to allow the latest status of the lamp (9) to be downloaded later by detecting the charge stored in the device. In order to avoid damage or tampering with the interior of the apparatus housing (41) in which the light source (9) is mounted, the sensor (1) may be attached to the apparatus housing by magnetic or adhesive fixtures (54) that are light tight to prevent ambient light from entering the light sensitive sensor. The sensor (1) can be connected to the lamp (9) using a light pipe (50) if the sensor cannot be attached directly to the device housing (41). Two or more sensors, each having a photosensitive sensor (or coupled with a suitable filter) sensitive to a different wavelength, may be used to detect a change in color of the indicator light (9).

Description

Passive optical sensor

The present invention relates to passive optical sensors, and in particular to sensors designed to allow automatic monitoring of human readable status indications such as alarm light turn-on or switch indication LEDs. Monitoring requires opening and closing relay contacts to generate an indication as to whether the machine is operating.

A power-on circuit can be used to monitor the circuit that powers the human-readable indicator light and create a switch output. A particular problem exists when it is desired to remotely monitor an existing "legacy" device which is not provided with the necessary interface to allow connection of the remote monitoring circuitry at the time of installation. Such equipment typically operates at mains voltage and making a connection will require suitably qualified personnel to disconnect the unit, open the unit (which may subject the operator to an electrical, chemical or other hazard), perform some rewiring, may also require converting the mains voltage to a safe voltage, and may require drilling the housing and fitting a connector. This may be impractical and for older unregistered devices there may be a risk of causing accidental modification to other internal devices within the same housing, such as moving other components or making unintentional changes to circuitry. The space within the housing may also be limited so as to not accommodate the necessary equipment, particularly where a step down transformer is required.

Such a monitoring system with human readable indications is shown in fig. 1. Illustratively, the equipment housing 41 contains a plurality of

dials

2, 3 and

indicator lights

9, 90, 91 to indicate the operating parameters of the equipment therein. As indicated by the warning signs 40 on the exterior of the housing, equipment within the housing may present hazards, such as high voltage or hazardous chemicals. It is often undesirable to open such equipment because of such hazards or the need to interrupt the operation of the equipment.

In this schematic view, the indicator light 4 is a light that is illuminated when the machine is running. In other devices, the lamp is lit when a fault occurs. Embodiments of the present invention are configured to monitor such indicator lights, for example, to trigger an alarm at a remote location if the machine is out of service. To achieve this, it is desirable to generate an output that acts as a switch. Existing passive components known as "light dependent resistors" are known for monitoring human readable lamps. However, under low light emitted by the human readable indication, the change in resistance is very small; typically 3 kilo ohms in the light and 14 kilo ohms in the dark. Such systems also require a power supply to apply a voltage across the resistor so that the change in resistance can be measured. The resistance typically varies linearly with the incident light, which results in a blurred output. The desired output behaves electronically more like an on-off switch, typically varying the resistance by many orders of magnitude, between less than 10 ohms (circuit complete) to several mega-ohms (circuit open).

According to the present invention there is provided a transducer for detecting the state of a light source, the transducer comprising a passive electro-optic sensor which generates an electrical potential in response to incident light, the passive electro-optic sensor being coupled to a field effect transistor which switches between distinct on and off states at a threshold electrical potential, wherein the field effect transistor is connected to an electrical storage device arranged to be charged by the photo-optic sensor and discharged to further circuitry. The states may be defined in terms of resistances, where one state is a resistance on the order of mega ohms and the other state is a resistance on the order of less than 100 ohms. The field effect transducer may be an enhancement mode metal oxide semiconductor field effect transistor (e-MOSFET) or a depletion mode metal oxide semiconductor field effect transistor (d-MOSFET).

The photosensor may be mounted in a housing shaped to enclose an indicator light and prevent light from other sources from reaching the photosensor. The housing may be fitted with a magnet to secure the housing to an instrument housing.

The photosensor may be optically coupled to a light pipe disposed to be coupled to an indicator light at its end remote from the sensor.

The photosensor may be responsive to a first light wavelength to generate a current and not to a second light wavelength, so that the sensor will respond to a color change of the indicator light. This can be set by fitting the photosensor with a filter. Alternatively, two or more transducers may be used, each transducer comprising a respective photosensor responsive to a different wavelength.

Because the sensor is passive, the use of a Field Effect Transistor (FET) allows the condition of the indicator light to be sensed efficiently without the need for a separate power source. FETs require much less energy to switch than bipolar junction transistors. The inventors have tested large area photodiodes, phototransistors for use with transistors, but none of them deliver sufficient resistance swing or current to act as an appropriate "switch".

In addition, the output of the FET switches quickly between a very low resistance (short circuit) and a very high resistance (open circuit). This is different from a Light Dependent Resistor (LDR), whose resistance continuously changes with changes in optical density. FETs also provide much larger relative resistance changes for small voltage changes, such as can be delivered by a photocell that uses only an indicator light as the light source.

In its simplest form, embodiments of the present invention provide a sensor and transmitter that does not require a power source or battery, but rather transmits data using only the power generated by light from the indication panel.

Other embodiments of the invention may be used to power a battery operated sensor and transmitter that is used to charge a rechargeable battery such that if the light subsequently stops being illuminated, the battery begins to discharge and the discharge current is used to send a message warning the operator of an abnormality.

The invention can also be used to drive a passive optical switch by providing a voltage difference (rather than a resistance) representing 0 or 1 (e.g., 0V and 5V).

To assist in understanding embodiments of the present invention, the characteristics of a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) will be briefly discussed with reference to fig. 2 and 3.

Fig. 1 is a schematic diagram of a MOSFET showing the key elements of the MOSFET: a substrate (B), and a drain terminal (D), a source terminal (S), and a gate terminal (G) (the latter three approximately correspond to the collector, emitter, and base, respectively, of a bipolar junction transistor). The gate G is separated from the substrate B by an insulating layer (N). A metal oxide semiconductor field effect transistor is a type of Field Effect Transistor (FET) and is most commonly fabricated by controlled oxidation of silicon. A mosfet has an insulated gate whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used to amplify or switch an electronic signal.

MOSFETs are very good electronic switches for controlling loads and in CMOS digital circuits, since they operate between their cut-off and saturation regions.

The main types of field effect transistors are called depletion mode and enhancement mode, which differ in whether the transistor is in the on-state or the off-state at zero gate-source voltage. By using either p-type or n-type semiconductors for the drain and source, one can further distinguish between the types of field effect transistors, these devices being referred to as PMOS and NMOS, respectively. The enhancement mode MOSFET is in an off state at zero gate-source voltage. NMOS devices may turn on by pulling the gate voltage higher than the source voltage, whereas PMOS devices may turn on by pulling the gate voltage lower than the source voltage. In most circuits, this means that the gate voltage of an enhancement mode MOSFET is pulled towards its drain voltage to turn it on.

The depletion mode MOSFET can be turned on by pulling the gate voltage below the source voltage. At zero gate-source voltage, the device is normally in a conducting state. Such devices are used as load "resistors" in logic circuits (e.g., in depletion mode NMOS logic). For an N-type depletion mode device, the threshold voltage may be about-3V, so the N-type depletion mode device can be turned off by pulling the gate up from minus 3V (in contrast, in an NMOS, the drain is more positive than the source). In PMOS, the polarity is opposite.

The mode may be determined by the sign of the threshold voltage: for an N-type FET, the threshold of the enhancement mode device is positive and the threshold of the depletion mode device is negative; for a P-type FET, the enhancement mode is negative and the depletion mode is positive.

An N-channel enhancement mode MOSFET (e-MOSFET) operates with a positive input voltage and has a very high input resistance (almost infinite) and thus can interface with almost any logic gate or driver capable of producing a positive output. Because of this very high input (gate) resistance, many different MOSFETs can be safely connected in parallel until the desired current handling capability is reached.

The operation of the enhancement mode MOSFET or e-MOSFET can best be described using the characteristic curves shown in fig. 3.

The minimum on-state gate voltage required to ensure that the MOSFET remains "on" while carrying the selected drain current can be determined from the V-I transfer curves shown in fig. 3, with each of the

curves

30, 31, 32, 33, 34 … … representing a plot of drain current Id versus drain/source voltage Vds for a corresponding value of gate/source voltage Vgs. It will be seen that for small Vgs values (e.g. below curve 36), the slope of these curves is linear and very flat, representing a very high resistance (drain current varies very slowly with voltage). However, for sufficiently high Vgs values (curve 30), a very steep gradient can be seen, indicating a very low resistance. Thus, inThere is a very abrupt transition between a very low resistance (actually a complete circuit) and a very high resistance (open circuit), making a MOSFET well suited as a switch responsive to voltage changes regardless of the magnitude of the change. Resistance R of the drain-source channel by applying an appropriate drive voltage Vgs to the gate of the FET_{DS (conduction)}It can vary from an "off resistance" (curve 36) of hundreds of kilo ohms, which is effectively open, to an "on resistance" (curve 30) of less than 1 Ω, which effectively acts as a short, so the MOSFET will act as a "single pole single throw" (SPST) solid state switch. This ability to turn the power MOSFET "on" and "off" allows the device to be used as a very efficient switch, where the switching speed is much faster than a standard bipolar junction transistor.

In an embodiment of the invention, the voltage Vgs is supplied by a photocell which is illuminated by an indicator lamp to be monitored and is used to switch another circuit.

FIG. 1, already discussed, shows a typical equipment cabinet to which the sensor of the present invention may be applied.

Fig. 2 has also been discussed and shows the general structure of a Metal Oxide Semiconductor Field Effect Transistor (MOSFET).

Fig. 3 shows the characteristics of an enhancement mode MOSFET.

Fig. 4 shows a first embodiment of a sensor according to the invention.

Fig. 5 shows a device comprising two sensors according to the embodiment of fig. 4.

Fig. 6 shows a second embodiment of a sensor according to the invention.

Fig. 7 shows a device comprising two sensors according to the embodiment of fig. 6.

Fig. 8 shows a first embodiment of the present invention.

Fig. 9 shows a second embodiment of the present invention.

Fig. 10 shows a sensor circuit illustrating the principles of the present invention.

Fig. 11 shows a variation of the sensor circuit of fig. 10 of the present invention.

Fig. 12 shows a first implementation of the principle.

Fig. 13 shows a second implementation of the principle.

Fig. 14 shows the circuit of fig. 12 in operation.

Fig. 15 shows the circuit of fig. 13 in operation.

FIG. 4 is a cross-sectional view through a sensor assembly of one embodiment of the present invention, and an indicator light mounted in the housing, the condition of which is to be monitored by the sensor. For clarity, the sensor is shown spaced from the housing to which it will be attached.

An indicator light 9 (which may be an incandescent light bulb, an LED, or any other indicator that is illuminated) is mounted in the housing 41. The lamp 9 is controlled by a circuit 42. The sensor assembly 1 is arranged to allow the status (lit/unlit) of the lamp 9 to be monitored remotely without having to penetrate the housing 41 or modify the electrical circuit 42.

The sensor itself comprises a photosensitive receiver 3, such as a CPC1831N solar cell. The receiver 3 is connected to the ground and source terminals of a field effect transistor FET 4/5 (shown in more detail in figures 2, 7 and 8). A suitable FET would be a 2N 7000N channel e-MOSFET. The FET is connected to a remote monitoring location by an electrical connection 46. The light-sensitive sensor 3 is surrounded by an opaque housing 47 on all sides except the side facing the lamp 9 to prevent ambient light from affecting the light-sensitive sensor 3. Preferably, the material is flexible so that a light tight seal can be formed with the device housing 41. The sensor assembly 1 is secured to the equipment housing by suitable attachment means 48, preferably by attachment means which do not damage the equipment housing 41 but will provide a secure attachment such that the light-tight housing 47 remains against the equipment housing 41 as indicated by arrow 49. Adhesive may be used or if housing 41 is made of steel, magnet 48 may be used to attach assembly 1. Attaching the unit from the outside by means of magnets means no risk, a low level of skill is required and there is no downtime for the monitored equipment.

The sensor assembly 1 is mounted on the device housing 41 such that light from the indicator light excites the light sensitive sensor 3, thereby causing the MOSFET 4 to operate to open the circuit 46 when the indicator light is not illuminated and to close the circuit when the indicator light is illuminated (or vice versa for an anti-logic sensor). An 8 volt solar cell allows the generation of energy from the light emitted by a typical indicator light, sufficient to turn the FET on/off. Therefore, the FET 4 cooperates with the switch 43 that controls the circuit 42 that lights up the lamp 9 to switch the circuit 46.

Fig. 6 shows an alternative configuration in which the light-sensitive sensor 3 may be located at a distance from the lamp. A light pipe (block of transparent material) is attached at one end to the photosensor 3 and fixed at its other end to the lamp 9 so that light from the lamp is coupled to the photosensor. As shown, the tube is fixed to the apparatus housing 41 and the sensor assembly 1 by a light absorbing adhesive tape 54, thereby restricting light from the outside from entering the photosensor 3.

In the embodiment of the invention shown in fig. 8, the electrical storage device 14 is shown as a capacitor, but a battery or batteries may be used. In this embodiment, a P-channel depletion mode MOSFET may be used. The device is normally turned on at zero gate-source voltage. The MOSFET requires a gate-source Voltage (VGS) to switch the device "off. Depletion mode MOSFETs are equivalent to "normally off" switches. The accumulator is charged by the energy generated by the photosensitive sensor 3. When the lamp 9 is extinguished, the battery starts to discharge.

Diodes

15, 16 or equivalent circuits direct current through the different circuits that operate the alarm during charging and discharging. One or both of the

diodes

15, 16 may be light emitting diodes which indicate the state of charge of the battery 14 and thus the status of the remote light 9.

Fig. 9 shows another embodiment in which the electrical storage device 14 is charged when the lamp 9 is lit (positive logic) or when it is not lit (negative logic). The state of charge of the battery 14 will depend on the number and length of events that the sensor 2 delivers power, and this can be interrogated when required by switching on (17) a measurement device such as a voltmeter or threshold detector circuit (18).

The remaining fig. 10-15 depict the logical operation of a positive logic implementation and a negative logic implementation, respectively, of the principles of the present invention.

1. Inverse logic

2. Positive logic

Fig. 10 illustrates a circuit for the inverse logic implementation of the embodiment of fig. 4 using an N-channel e-MOSFET (4), while fig. 11 illustrates a modified circuit for the positive logic variant 2 of the embodiment of fig. 4 using a P-channel e-MOSFET (5). In both figures, switch connection (6), ground connection (7) and power supply (8) connection are shown-note that in the inverse logic embodiment (fig. 10), the power supply connection 8 is connected to the gate terminal of the FET and the source terminal is connected to ground, whereas in the positive logic embodiment of fig. 11, these connections are reversed. In both cases, the switch output 6 is connected to the drain terminal D.

Fig. 12 shows the structure of the inverse logic sensor 1 switching controller 10, and fig. 13 shows the same inverse logic sensor 1 in another structure. In these figures, a controller 10 is shown as a built-in pull-up/pull-down resistor 11 and driver/processor/transmitter circuitry 12. In both embodiments, the controller 10 is powered up when the LED/lamp (9) is on. In fig. 13, the supply voltage (8) (delivered from the photocell 3) is used to power up the components of the controller 10, while in fig. 12 it switches the controller with an independent power supply VCC.

Fig. 14 shows how the two states of the LED (9) affect the output of the anti-logic sensor (1). The upper diagram shows the sensor system and the equivalent circuit when the lamp 9 is not lit, and the lower diagram shows the sensor system and the equivalent circuit when the lamp is lit.

As the voltage between the gate pin (G) and the source pin (S) varies, the potential at the drain pin (D) connected to the switch output 6 switches between ground and the highest (supply) voltage Vs.

In the unlit condition, the photocell 3 generates no voltage between the gate terminal G and the source terminal S, driving the potential at the switching output 6 high. This is equivalent to a high resistance 13 (or open switch) between the source pin (connected to ground) and the switch output, driving the input to the processor high and generating a logic "1".

Igniting the lamp 9 raises the potential at the gate terminal close to the potential at the drain terminal (switch connection 6) driving the potential at the switch output 6 low, which lowers the potential between the processor input and ground, equivalent to reducing the resistance 13 or closing the switch, generating a logic "0".

Fig. 15 shows how the two states of the LED (9) affect the output of the positive logic sensor (2). The upper diagram shows the sensor system and the equivalent circuit when the lamp 9 is not lit, and the lower diagram shows the sensor system and the equivalent circuit when the lamp is lit.

As the voltage between the gate pin (G) and the drain pin (D) varies, the potential at the drain pin (D) connected to the switch output 6 switches between ground and the highest (supply) voltage Vs.

In the unlit condition, the photocell 3 generates a high voltage between the gate terminal G and the drain terminal D, driving the potential at the gate (switch) output 6 high. This is equivalent to a high resistance 11 between the gate pin (connected to ground) and the switch output (or opening the switch), driving the input to the processor low and generating a logic "0".

Igniting the lamp 9 lowers the potential at the gate terminal close to the potential at the drain terminal (switch connection 6) driving the potential at the switch output 6 low, which lowers the potential between the processor input and ground, equivalent to reducing the resistance 13 or closing the switch, generating a logic "1".

Fig. 8 fig. 9 the embodiment can be modified to monitor lights that change color to indicate different operating states by arranging the sensor 3 coupled to the indicator light 9 to be sensitive to wavelengths corresponding to one operating state but not the other. Preferably, as shown in fig. 5 and 7, a plurality of sensors are used, each sensitive to a respective wavelength, since a single sensor does not distinguish between a failure of the indicator and illumination of a color to which the sensor is insensitive ("false negative"). The different sensitivities can be selected by using photosensitive sensors 3 that respond to the appropriate wavelength or by using suitable filters 52, 53 as shown in fig. 5 or by using

light pipes

50, 51 with different optical characteristics as shown in fig. 7.

Claims

1. A transducer for detecting the state of a light source, the transducer comprising a passive electro-optic sensor generating an electrical potential in response to incident light, the passive electro-optic sensor being coupled to a field effect transistor which switches between distinct switching states at a threshold electrical potential, wherein the field effect transistor is connected to an electrical storage device arranged to be charged by the photo-sensor and discharged to further circuitry.

2. The transducer according to claim 1, in which the states are defined in terms of resistance, wherein one state is a resistance of the order of mega ohms and the other state is a resistance of the order of less than 100 ohms.

3. The transducer according to claim 1 or claim 2, wherein the field effect transistor is an enhancement mode metal oxide semiconductor field effect transistor.

4. The transducer according to claim 1 or claim 2, wherein the field effect transistor is a depletion mode metal oxide semiconductor field effect transistor.

5. A transducer according to any of the preceding claims, electrically coupled to a controller having a transistor switch responsive to the output of the field effect transistor.

6. A transducer according to any of the preceding claims, wherein the photosensor is mounted in a housing shaped to enclose an indicator light and prevent light from other sources from reaching the photosensor.

7. The transducer according to claim 6, wherein the housing is fitted with a magnet to secure the housing to an instrument housing.

8. The transducer of claim 1, claim 2, claim 3, claim 4 or claim 5, wherein the photosensor is optically coupled to a light pipe disposed to be coupled to an indicator light at its end remote from the sensor.

9. The transducer of any of the preceding claims, wherein the photosensor is responsive to a first light wavelength to generate an electrical current and is not responsive to a second light wavelength.

10. The transducer according to claim 8 or claim 9, wherein the photosensor has a filter.

11. An assembly comprising two or more transducers, each according to claim 8, claim 9 or claim 10, wherein the respective photosensors are responsive to different wavelengths.